In many technical domains (e.g. microelectronic or photovoltaic technology) materials, such as e.g. silicon, germanium or sapphire, are often needed in the form of thin discs and plates (so-called wafers). As standard, such wafers are currently produced by sawing from an ingot, relatively large material losses (“kerf loss”) occurring. Since the source material used is often very expensive, great efforts are being made to produce such wafers with less material consumption and so more efficiently and inexpensively.
For example, with the currently normal methods almost 50% of the material used is lost as “kerf loss” when producing silicon wafers for solar cells alone. Considered globally, this corresponds to an annual loss of more than 2 billion euros. Since the cost of the wafer makes up the greatest percentage of the cost of the finished solar cell (over 40%), the cost of solar cells could be significantly reduced by making appropriate improvements to the wafer production.
Methods which dispense with the conventional sawing and can separate thin wafers directly from a thicker workpiece, e.g. by using temperature-induced stresses, appear to be particularly attractive for this type of wafer production without kerf loss (“kerf-free wafering”). These include in particular methods as described e.g. in PCT/US2008/012140 and PCT/EP2009/067539 where a polymer layer applied to the workpiece is used in order to produce these stresses.
In the aforementioned methods the polymer layer has a thermal expansion coefficient that is higher by approximately two orders of magnitude in comparison to the workpiece. Moreover, by utilising a glass transition a relatively high elasticity modulus can be achieved in the polymer layer so that sufficiently large stresses can be induced in the polymer layer workpiece layer system by cooling in order to enable the separation of the wafer from the workpiece.
Upon separating a wafer from the workpiece, in the aforementioned methods polymer still adheres to a respective side of the wafer. The wafer bends here very strongly towards this polymer layer, and this makes it difficult to execute the separation in a controlled manner, and may lead e.g. to variations in the thickness of the separated wafer. Moreover, the strong curvature makes subsequent processing difficult and may even lead to the wafer shattering.
When using the methods according to the previous prior art, the wafers produced generally have respectively larger thickness variations, the spatial thickness distribution often showing a pattern with four-fold symmetry. The total thickness variation seen over the entire wafer (“total thickness variation”, TTV) is often more than 100% of the average wafer thickness when using the previous methods (a wafer with an average thickness of, for example, 100 micrometers, that is e.g. 50 micrometers thick at its thinnest point and 170 micrometers thick at its thickest point has a TTV of 170-50=120 micrometers, which corresponds to a total thickness variation of 120% relative to its average thickness). Wafers with these strong thickness variations are unsuitable for many applications and the thickness variations lead to problems or losses in efficiency due to uneven contact when subsequently processing the wafer. Moreover, with the most frequently occurring four-fold thickness distribution patterns, the regions with the greatest variations unfortunately lie in the middle of the wafer where they cause the greatest disruption.
Moreover, in the method according to the current prior art, undesirable oscillations in the layer systems involved occur during the break propagation when the separation itself is taking place, and these oscillations have a negative impact upon the development of the break front and may in particular lead to significant thickness variations of the separated wafer.
In addition, with the previous methods it is difficult to ensure reproducibly good heat contact over the entire surface of the polymer layer. Locally insufficient heat contact may, however, lead to undesirable significant local temperature variations in the layer system due to the low thermal conductivity of the polymers used, and this on its part has a negative impact upon the controllability of the stress fields produced and so upon the quality of the wafers produced.
Finally, there is also a requirement for a method in which the coated workpieces can easily be mechanically fixed during the cooling process (optionally also exerting external forces to additionally affect the separation process), and where the wafers are easy to handle after separation. According to the current prior art many wafers shatter after separation because the relatively heavy polymer layer adhering to the wafer becomes soft after heating and so can no longer support the brittle, thin wafer sufficiently.